When light rays enter a reflective image display and are reflected, there is an average lateral displacement of the entering and exiting light rays. Depending on the angle of the incident light ray, the type of reflective surface and the total distance that the light ray travels, the degree of lateral displacement can vary greatly. The degree of lateral displacement may be problematic. Lateral displacement is problematic if a light ray enters the display through one color sub-pixel and exits out of an adjacent sub-pixel of a different color. More specifically, if a light ray enters a red sub-pixel but exits a green sub-pixel in a reflective color display comprising of an RGB (red-green-blue) color filter array. Firstly, a loss of resolution may result as the edges of the pixels are blurred. Secondly, a loss of efficiency may result since the exiting light ray can end up having a lower intensity. The severity of these two effects depends on the degree of average lateral displacement. If the lateral displacement is large, then the effects will be significant. Conversely, if the lateral displacement is small, then the effects will be insignificant. These effects are present both in monochrome and color displays. The loss of resolution may only matter if the blurring is visible. If, for example, the pixels or sub-pixels are very small then even if there is some blurring present it may not affect the apparent resolution of the display as it is not observable by the naked eye. The loss of efficiency, though, is a potentially more serious problem. Even if the pixels are not visible to the naked eye, the efficiency loss has an averaging effect and will reduce the apparent contrast of the display. While this is true both for monochrome and color image displays, it is a more challenging problem for color displays. This is because the sub-pixels are smaller and therefore the tolerance to lateral displacement is not nearly as high. In other words, it's more important to minimize the lateral displacement for a color display that has smaller sub-pixels than for a monochrome display that typically has pixels that are approximately three times larger. In a color reflective image display it is preferred that in order to maintain a high quality color image the light that enters the display through a color sub-pixel is reflected back to the viewer through the same color sub-pixel. It is essential that the sites within the reflective image display that reflects light are properly aligned or registered with the color sub-pixels to be capable of creating a high quality image.
Another desirable quality of reflective image displays comprising of light absorbing electrophoretically mobile particles suspended in an optically clear fluid is that the particles are distributed with approximately equal density throughout the display. This is so the display appears substantially uniform in appearance during operation of the display. It is also desirable that the appearance of the display maintains a high level of quality and efficiency throughout the life of the display. One method to do this is to micro-segregate the particles to prevent lateral migration.